Systems of this general type have been used in the past for automatically tracking the sun from a vehicle; see U.S. Pat. No. 3,084,261. More recent uses have been in the field of keeping track of the head motion of a person operating a vehicle, specifically the pilot of an aircraft holding a target in sight. Thus, as described for example in U.S. Pat. No. 3,917,412, a pilot's helmet may be equipped with two radiation emitters irradiating a pair of detectors whose outputs, fed to a computer, enable the determination of the pilot's line of sight paralleling the line of intersection of two planes, each of these planes being defined by the two emitters and by the location of the respective detector.
In the case of a single point source, its direction as seen from a sensing device can also be established with the aid of two slits perpendicular to each other in a mask otherwise opaque to the emitted radiation, these slits being traversed by flat beams or sheets of incident rays lying in mutually orthogonal planes whose orientation can be ascertained with the aid of two elongate radiation detectors respectively intersected by these beams. See, in this connection, U.S. Pat. No. 3,951,550. A modification of that system, described in French Pat. No. 2,322,356, utilizes a V-shaped slit training two flat beams from the same source upon a single elongate radiation detector whose points of intersection with these beams indicate the direction of the source.